1. Field of the Invention
The invention relates to compositions based on organopolysiloxanes, to a process for preparing them, and to their use as defoamers.
2. Description of the Related Art
In numerous liquid systems, particularly aqueous systems which contain surface-active compounds either as desired or undesired constituents, it is possible for problems to occur as a result of foaming if these systems are contacted more or less intensively with gaseous substances. Such contact may occur, for example, during gassing of wastewater, during intensive stirring of liquids, during distillation, in washing or coloring operations, or during dispensing procedures. Foaming can be controlled by mechanical means or through the addition of defoamers. Siloxane-based defoamers have been found particularly appropriate.
Defoamer formulations based on siloxanes may be prepared, for example, in accordance with DE-B 1519987, by heating hydrophilic silica in polydimethylsiloxanes. In order to improve the activity of defoamer formulations of this kind, numerous proposals have been made. For instance, U.S. Pat. No. 2,632,736 provides improved defoamer formulations by carrying out partial crosslinking of the polysiloxane by heating in the presence of atmospheric oxygen. Similarly partly-crosslinked or branched polysiloxanes are described in EP-A 163 541 and EP-A 434 060. This route has the disadvantage that an additional, cost-consuming reaction step is necessary and that the defoamer formulations thus prepared, owing to their viscosity or even viscoelasticity, generally have poor handling properties.
Through the use of catalysts which promote the reaction between the filler and the silicone it is possible to improve the activity of the defoamers economically. DE-A 1545185 describes defoamer formulations having improved activity in alkaline media by preparation in the presence of acidic catalysts such as anhydrous Lewis acids, an example being solid, mortar-ground aluminum chloride. EP-A 163 541 describes the preparation of defoamer formulations in the presence of KOH as catalyst, the potassium hydroxide being reacted in an additional, costly and inconvenient step with polysiloxanes to first form a potassium siloxanolate, which is then added to the mixture of polysiloxanes and silica. DE-A 10001945 also describes the preparation of defoamer formulations with basic catalysts, which are used directly or as reaction products with siloxanes. The preparation of basic siloxanolates is costly and inconvenient, and the handling of anhydrous alkalis or Lewis acids such as aluminum chloride, is inconvenient in comparison to handling aqueous solutions, and more costly and risky as well. A further disadvantage of the use of catalysts is that with catalysts it is generally not possible to ensure the purity required for applications in pharmaceutical products or in foods, since the catalysts and their reaction products remain in the defoamer formulation.